When light scatterers like biological tissues are illuminated with light, the resulting, rectilinearly propagating light could be extracted to some extent in a 180.degree. face-to-face system. As yet, however, the spatial resolving power is not good enough.
A difference in the spatial resolving power between light and X-rays cannot be made up for as yet. The use of light, esp. near infrared rays, however, will be able to construct images of tissue's oxygen concentrations from hemoglobin in the blood. These will provide information different from that obtained with other techniques such as
Now let us assume an object 0 shown in FIG. 22 does not contain too much scatterers and is relatively close to transparency. Then, light having a specific wavelength component selected through a filter 340 is directed onto the object 0 from a ring type of slit 341 placed at the focal position of a lens L.sub.1, so that the enlarged image can be focused onto a plane P through an objective L.sub.2 for observation. The use of the ring type of slit 341 located at the focal point of the lens L.sub.1 is tantamount to irradiating the object 0 with light in every direction, as shown in FIG. 23, so that images I.sub.1, I.sub.2, and so on, of the object 0 in the respective directions can be observed at once.
Given a 3 to 5-cm thick tissue, we can detect light transmitted through it. This means that "opto-radiography" can be used for diagnosis. The mammas are relatively uniform in tissue and easy to transmit light, and the transmitted light is easily detectable (at a thickness up to about 3 cm) because of their form. Thus, this technique has long been used for the diagnosis of breast cancer under the name of "Diaphanography" or "Lightscanning". One such conventional diagnostic system will now be explained with reference to FIG. 24.
The construction of a conventional system for obtaining a light absorption distribution image is illustrated in FIG. 24, wherein reference numeral 401 stands for a scan head, 403 the human body, 405 a video camera, 407 an A/D converter, 409 a near infrared light frame memory, 411 a red light frame memory, 413 a processor, 415 a color conversion processor, 417 an encoder keyboard, 419 a D/A converter, 412 a printer, 423 a TV monitor and 425 a video tape recorder. The spot of the human body to be inspected, e.g. the mamma is irradiated and scanned alternately with red light (strongly absorbed in hemoglobin in the blood in particular) and near infrared light (absorbed in the blood, fluids, fat, etc.) by the scan head 401 through a light guide. As shown, the spot is illuminated with light from below. As a result, the mamma glows brightly, and the transmission image is picked up by the video camera 401. That image is converted through the A/D convertor 407 into digital signals, and the near infrared light and red light are fed in the frame memories 409 and 411, respectively, through a digital switch. The ratio of intensities of near infrared light and red light is then computed in the processor 413, and is further converted into analog signals by color conversion processing. The resulting light absorption distribution image is finally observed through a printer, a TV monitor or a video tape.
This system's resolving power is not good enough, because the light leaving the scan head 401, which is not parallel light, diverges through the tissue (the mamma), as much it would be illuminated with the light from a flashlight, and is received by such a two-dimensional receptor as a video camera.
An example of conventional illuminator/receptor systems collimated so as to make improvement in this regard will now be explained with reference to FIG. 25.
FIG. 25 is a diagrammatic sketch illustrating the construction of a conventional unit for obtaining light absorption distribution images using a collimated illuminator/receptor system.
In this example, laser light emanating from a light source is guided onto an object 435 to be inspected through an optical fiber 433 for illumination, and the transmitted light is picked up by a fiber collimator 437 and fed into a receptor 443 wherein it is converted into electrical signals, which are in turn processed in a computer 451 through a preprocessing circuit 445, an A/D converter 447 and an interface 449. In this case, scanning is carried out while the optical fibers 433 for illumination are synchronized with the fiber collimator 437 for detection by a motor 439, thereby obtaining light absorption distribution images of the respective spots of the object, which are in turn observed on a monitor 453.
It is noted that the red light source used is a 633-nm He-Ne laser and the near infrared light source used an 830-nm semiconductor laser. With this diagnostic unit, Jobsis and coworkers reported in 1977 that they succeeded in detecting light transmitted through the heads of cats or humans illuminated with near infrared light and the amount of that transmitted light was found to vary depending upon the animals' respiration. With near infrared light having a wavelength of 700 to 1500 nm and a tissue specimen, nearly the size of the head of a cat, the transmitted light could be well detected at a dose of about 5 mill. This dose is greatly safe, because it is about 1/50 or less of the present safety standards for laser, or about 1/10 of near infrared light to which we are now being exposed at the seaside.
Incidentally, when living bodies, etc. are irradiated with light, the transmitted light is absorbed and scattered by the specimens.
FIG. 26 is the Twersky's scattering theory curves that clarifies the relationship between the absorbance of an erythrocite-suspending fluid and the concentration of hematocrit, and shows the intensity, scattered and absorbance components of the transmitted light obtained by illumination of laser light having a wavelength of 940 nm.
As can be seen from FIG. 26, the transmitted light has the large scattered component superposed on the absorbance component. The scattered component, because of being directionality-free, has the property of coming to contain scattered light from various spots and making optical tomograms blurry. For this reason, mere detection of the transmitted light does not allow the absorbance component, that is the required information, to be detected with high accuracy.
FIG. 27 is a diagrammatic sketch illustrating the optical properties of such a specimen as a living body.
Referring here back to FIG. 22, the object 0 contains no scattering component. In other words, what is observed in FIG. 22 is, so to speak, an originally visible object. A specimen 460 of FIG. 27, that is to be actually observed, should be essentially considered to be made up of a Rayleigh scatterer 460a smaller than the wavelength of light; a Mie scatterer 460b nearly the size of the wavelength of light; a light transmission information carrier 460c that is the target to be observed and absorbs light; a diffuser 460d that diffuses light; a diffraction grating 460e that gives rise to random diffraction; and so on. When such a specimen is irradiated with coherent plane waves through a laser optical system 461, light leaving it comes to contain, in addition to the transmitted light, the Rayleigh scattered, Mie scattered, diffused, randomly diffracted and other forms of light. So far, it has been impossible to detect only the light transmitted through the information carrier 460c from all such forms of light.
FIG. 28 is a diagrammatic sketch illustrating a Fresnel diffraction wave produced by a sinusoidal grating having a finite aperture.
As a plane wave is directed onto the finite aperture, side bands 471 and 472 occur in addition to transmitted light 470. With a random diffraction grating, therefore, difficulty will be involved in detecting and observing the transmitted light with high sensitivity owing to the influence of the side bands.
FIGS. 29A and 29B are diagrammatic sketches illustrating a luminance distribution found on a plane of view located in opposition to a random scatterer, when it is illuminated with coherent light.
When such a scatterer as a living body is irradiated with such coherent light as laser light, a random diffraction image appears on the plane of view, as shown in FIG. 29a. Then, the transmitted light through the scatterer is focused by a lens L onto the plane of view, as shown in FIG. 29b. However, it is impossible to inspect an image of the spot of a living body or the like to be observed with high resolution, because a random diffraction image is superposed on that image.
FIGS. 30A and 30B are diagrammatic sketches showing a luminance distribution of reflected light that depends on what state a diffuse reflection plane is in, with FIGS. 30a and 30b taking the form of polar and rectangular coordinates, respectively.
In FIGS. 30A and 30B, J stands for a luminance distribution of light reflected from a perfect diffusion plane, G denotes a luminance distribution of light reflected from a glossy plane, and P indicates a luminance distribution of light reflected from a dim plane. On the glossy plane the luminance distribution shows a peak converging sharply in the predetermined direction, while on the dim plane the luminance distribution is diverging. Thus, it turns out that the luminance distribution varies depending upon what state the plane is in and that the accuracy of observation making use of reflected light is largely governed by what state the plane is in.
As mentioned above, if tomographic images are observed with coherent light, it is then impossible to view them with high resolution, since the required information light is buried in various forms of scattered light.
Having been accomplished with a view to providing a solution to the above-mentioned problems, the present invention has for its object to provide a heterodyne receptor system capable of detecting the required information light from many scattered components with the use of a short receptor element, even when information light is buried therein, whereby optical tomograms of a living body or the like can be imaged, and an arrangement for imaging optical tomograms.